We have shown at the levels of isolated cell, isolated tissue and the intact heart in vivo that increased microtubule network density is one cause of contractile dysfunction in high wall stress right or left ventricular hypertrophy. We then showed that increased affinity of microtubule-associated protein 4 [MAP4] for microtubules is the primary determinant of hypertrophy-related microtubule network sta- bilization and densification, and is thereby responsible for both the contractile dysfunction and associ- ated defects in microtubule-based intracellular trafficking that we have also described in this setting. MAP4 phosphorylation status appears to determine MAP4 affinity for microtubules. Therefore, this application intends to characterize the persistent changes in site-specific MAP4 phosphorylation status in an animal model of pathological pressure overload hypertrophy, with a parallel model of physiologi- cal volume overload hypertrophy serving as a control for any effects of growth per se. We will then delineate the regulation of MAP4 phosphorylation by examining relevant kinases and phosphatases. We believe that the successful execution of the proposed research will likely generate important new information to help understand the cellular and molecular mechanisms underlying cardiac hypertro- phy and failure. In addition, while our own end point of interest is the microtubule, the persistent increase in cardiac phosphatase activity and its regulation as shown in our preliminary data is almost certainly of more general importance to established hypertrophy- and failure-related disease mecha- nisms, many of which involve phosphorylation-dependent alterations of structural and regulatory pro- teins of the sarcoplasmic reticulum, the myofibril, and the extra-myofilament cytoskeleton. Thus, this work may serve as an example to help conceptualize the likely mechanistic convergence of a number of heretofore apparently disparate abnormalities of the hypertrophied and failing heart.